Plug in your device via USB cable and open terminal window as an administrator (Windows). Boot device into bootloader by executing

adb reboot bootloader

You can also boot to bootloader by holding volume down button when powering on the device (or when selecting to boot to Android if you have a dual-boot device).

Execute in terminal

fastboot oem unlock

Unlocking bootloader doesn’t erase userdata on this device.

Installing TWRP:

Change into the directory you saved the TWRP recovery image, boot to bootloader as above, and execute

fastboot flash recovery recovery-twrp3020-cht_cr_mrd_w.img

You can also just boot TWRP recovery image by executing

fastboot boot recovery-twrp3020-cht_cr_mrd_w.img

You can boot to TWRP by executing ‘adb reboot recovery’. It’s also possible to boot to recovery by booting to bootloader first (as above), using volume keys to select ‘recovery’ option and using power key to confirm.